1. Field of the Invention
The invention relates to a drive control apparatus for a vehicle and a control method thereof, and more particularly to a control of a lock-up clutch.
2. Description of the Related Art
A drive control apparatus for a vehicle is known, which includes (a) an engine which generates power using combustion of fuel; (b) an automatic transmission which can automatically change a gear ratio; (c) a hydrodynamic power transmission device which transmits an output of the engine to the automatic transmission via fluid, and in which an input side and an output side can be directly coupled using a lock-up clutch; (d) a fuel cut device which stops fuel supply to the engine when a vehicle is coasting with a throttle valve of the engine being fully closed, and a predetermined fuel cut condition is satisfied; and (e) a lock-up engagement device which engages the lock-up clutch when a predetermined lock-up engagement condition is satisfied. An example of such a drive control apparatus for a vehicle is disclosed in Japanese Patent Laid-Open Publication No. 9-53718. In the drive control apparatus for a vehicle, a lock-up clutch is engaged when a vehicle is coasting, which increases an engine rotational speed and enlarges a fuel cut region (a vehicle speed range), thereby improving fuel efficiency.
In such a drive control apparatus for a vehicle, when fuel supply is restarted from a fuel cut state according to a driver's accelerator operation (a driver's output request) and an engine output is increased, an operating state of the engine is changed from an engine brake state to a driving state. Therefore, a shock may occur due to a change in a driving force of the engine. In the case where such a shock occurs when an amount of accelerator operation is relatively small and tip-in acceleration which is gradual acceleration is performed, riding comfort may become poor and the driver may feel uncomfortable.
In order to solve the problem, it is possible to perform a smoothing processing for smoothing a change in the engine output, and further a change in the driving force, by performing control for delaying ignition timing of the engine or the like. However, in a region where there is a possibility that knocking will occur at a relatively low vehicle speed, the smoothing processing is restricted by knocking prevention control, i.e., engine control for preventing knocking, which makes it difficult to fully prevent a shock. Particularly, in the case of an engine in which a knocking limit is low, it is extremely difficult to perform both the smoothing processing for preventing a shock and the knocking prevention control.
Meanwhile, a droning noise may occur at a preset engine rotational speed region due to resonance between vibration of a driving system such as the engine and a vehicle body. In such a droning noise occurrence region, occurrence of a droning noise is suppressed by disengaging the lock-up clutch, or by correcting a shift map (a shift condition) for the automatic transmission such that the engine rotational speed does not constantly remain in the droning noise occurrence region. However, when the lock-up clutch is disengaged, fuel efficiency deteriorates due to transmission loss in the hydrodynamic power transmission device. When the shift map is corrected, fuel efficiency and running performance may deteriorate.